The recent performance upgrade of computers and networks continues to increase distribution of digital contents composed of image data such as still images or videos, audio data, and so on. Particularly, since it is easy to edit the digital image, unauthorized falsification has been recognized as a social problem. Under such circumstances, attention has been drawn to the electronic watermark technology of embedding specific information in multimedia data such as images and audio, with little effect on a total information amount of data (cf. Patent Document 1).
The electronic watermark technology can prevent unauthorized copy and data falsification and also permits embedding of right management information such as a notice of copyright, as well as the fact of data falsification. The electronic watermark technology employs a variety of processing methods depending upon objects for embedding of watermark information in an original image (digital image) and an example thereof will be described as to a case of a still image, using FIG. 1. FIG. 1 is a conceptual diagram for explaining a general application example of the electronic watermark technology to a digital image.
First, an original image I (still image) is subjected to frequency transformation by Fourier transform and the resultant transformed data is manipulated with “key” information such as an embedded position or intensity to insert pulsed watermark information S in the transformed data. Then the transformed data with the watermark information S therein is subjected to inverse Fourier transform to obtain an image IS in which the watermark information S is embedded. Since the watermark information S is spread over the entire image in the process of inverse Fourier transform, the resultant image IS and the original image I are apparently indistinguishable.
On the other hand, the resultant image IS will exist in a falsification-vulnerable environment 100. Specifically, it will be distributed through a network 101 to third parties and recorded and stored in a recording device 102 such as a hard disk (including recording media such as CD and DVD distributed in the market). For detecting falsification of an image I′S having existed in the falsification-vulnerable environment 100, electronic watermark information S′ embedded therein is extracted from the acquired image I′S by dedicated software and this extracted watermark information S′ is compared with the watermark information S previously embedded, so as to permit a determination to be made on whether falsification is made in fact in the convertible environment 100.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-341632